With the advancement of information communication technologies, various wireless communication technologies have recently been developed. Among the wireless communication technologies, a wireless local area network (WLAN) is a technology whereby Internet access is possible in a wireless fashion in homes or businesses or in a region providing a specific service by using a portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc.
Ever since the institute of electrical and electronics engineers (IEEE) 802, i.e., a standardization organization for WLAN technologies, was established in February 1980, many standardization works have been conducted. In the initial WLAN technology, a frequency of 2.4 GHz was used according to the IEEE 802.11 to support a data rate of 1 to 2 Mbps by using frequency hopping, spread spectrum, infrared communication, etc. Recently, the WLAN technology can support a data rate of up to 54 Mbps by using orthogonal frequency division multiplex (OFDM). In addition, the IEEE 802.11 is developing or commercializing standards of various technologies such as quality of service (QoS) improvement, access point protocol compatibility, security enhancement, radio resource measurement, wireless access in vehicular environments, fast roaming, mesh networks, inter-working with external networks, wireless network management, etc.
In the IEEE 802.11, the IEEE 802.11b supports a data rate of up to 11 Mbps by using a frequency band of 2.4 GHz.
The IEEE 802.11a commercialized after the IEEE 802.11b uses a frequency band of 5 GHz instead of the frequency band of 2.4 GHz and thus significantly reduces influence of interference in comparison with the very congested frequency band of 2.4 GHz. In addition, the IEEE 802.11a has improved the data rate to up to 54 Mbps by using the OFDM technology. Disadvantageously, however, the IEEE 802.11a has a shorter communication distance than the IEEE 802.11b. Similarly to the IEEE 802.11b, the IEEE 802.11g implements the data rate of up to 54 Mbps by using the frequency band of 2.4 GHz. Due to its backward compatibility, the IEEE 802.11g is drawing attention, and is advantageous over the IEEE 802.11a in terms of the communication distance.
The IEEE 802.11n is a technical standard relatively recently introduced to overcome a limited data rate which has been considered as a drawback in the WLAN. The IEEE 802.11n is devised to increase network speed and reliability and to extend an operational distance of a wireless network. More specifically, the IEEE 802.11n supports a high throughput (HT), i.e., a data processing rate of up to 540 Mbps or higher, and is based on a multiple input and multiple output (MIMO) technique which uses multiple antennas in both a transmitter and a receiver to minimize a transmission error and to optimize a data rate.
In addition, this standard may use a coding scheme which transmits several duplicate copies to increase data reliability and also may use the OFDM to support a higher data rate.
A basic access mechanism of an IEEE 802.11 medium access control (MAC) mechanism is a carrier sense multiple access with collision avoidance (CSMA/CA) combined with binary exponential backoff. The CSMA/CA mechanism is also referred to as a distributed coordinate function (DCF) of the IEEE 802.11 MAC, and basically employs a “listen before talk” access mechanism. In this type of access mechanism, a station (STA) listens a wireless channel or medium before starting transmission. As a result of listening, if it is sensed that the medium is not in use, a listening STA starts its transmission. Otherwise, if it is sensed that the medium is in use, the STA does not start its transmission but enters a delay period determined by the binary exponential backoff algorithm.
The CSMA/CA mechanism also includes virtual carrier sensing in addition to physical carrier sensing in which the STA directly listens the medium. The virtual carrier sensing is designed to compensate for a limitation in the physical carrier sensing such as a hidden node problem. For the virtual carrier sending, the IEEE 802.11 MAC uses a network allocation vector (NAV). The NAV is a value transmitted by an STA, currently using the medium or having a right to use the medium, to another STA to indicate a remaining time before the medium returns to an available state. Therefore, a value set to the NAV corresponds to a period reserved for the use of the medium by an STA transmitting a corresponding frame.
One of procedures for setting the NAV is an exchange procedure of a request to send (RTS) frame and a clear to send (CTS) frame. The RTS frame and the CTS frame include information for reporting upcoming frame transmission to receiving STAs and thus capable of delaying frame transmission of the receiving STA.
The information may be included in a duration filed of the RTS frame and the CTS frame. After performing the exchange of the RTS frame and the CTS frame, a source STA transmits an actual frame to be transmitted to a destination STA.
FIG. 1 is a diagram showing an IEEE 802.11 MAC architecture including the aforementioned DCF.
Referring to FIG. 1, a point coordination function (PCF) and a hybrid coordination function (HCF) are provided by using a DCF service. The HCF includes an enhanced distributed channel access (EDCA) and an HCF controlled channel access (HCCA). The HCF does not exist for an STA which does not support quality of service (QoS), whereas both of the DCF and the HCF exist for an STA which support QoS. The PCF is an arbitrary function for all STAs.
Meanwhile, the IEEE 802.11n standard specifies a power-saving multi-poll (PSMP) protocol. In an operation based on the PSMP protocol, a high throughput (HT) access point (AP) allocates a downlink transmission time (DTT) and an uplink transmission time (UTT) to each HT non-AP STA (hereinafter, HT STA) associated with the HP AP or to HT STAs of a specific group, and the HT STA communicates with the HT AP only during the DTT and UTT allocated to the HT STA.